(1) Field of the Invention
This invention relates to a polypropylene-base resin composition having excellent paintability and high stiffness, impact resistance and molding fluidity.
(2) Description of the Prior Art
Polypropylene resin has now found widespread commercial utility owing to its low specific gravity, high stiffness and superb heat and chemical resistance. In the beginning, polypropylene resin was however unsuitable for use at low temperatures, because of its poor impact resistance at such low temperatures. Polypropylene resins of the impact resistant grade, which are obtained by copolymerizing ethylene and propylene, have been developed and put on the market as polypropylene resins usable at low temperatures. However, polypropylene resins have recently been required to exhibit still higher performance and the impact resistance of such a degree as the impact resistant grade has been found to be insufficient in some applications. In the field of automobile bumpers, bumper skirts, trims, etc. for example, there is a demand for resins having mutually-contradictory properties such as high stiffness, high heat resistance and easy paintability as well as, at the same time, high impact resistance. High molding fluidity is further required especially in the case of bumpers, reflecting the recent move toward larger moldings.
Various approaches have thus been proposed to improve the impact resistance and paintability of propylene resins. Japanese Patent Laid-open No. 47344/1982 discloses a polypropylene-base resin composition consisting of 50-97 wt.% of a crystalline ethylene-propylene block copolymer and 50-3 wt.% of an elastic ethylene-propylene-diene terpolymer having a Mooney viscosity(ML.sub.1+4, 100.degree. C.) of 70-150. Japanese Patent Laid-open No. 55952/1982 discloses a polypropylene composition suitable for use in the fabrication of bumpers, which composition consists of 55-65 wt.% of a crystalline ethylene-propylene block copolymer having an ethylene content of 5-10 wt.%, polypropylene units insoluble in boiling n-heptane of 97 wt.% or more, units soluble in para-xylene of room temperature having an intrinsic viscosity of 3-4 as measured in decaline (135.degree. C.), and a melt flow index of 2-10; 30-35 wt.% of an amorphous ethylene-propylene copolymer having an intrinsic viscosity of 2.0-3.5 as measured in decaline (135.degree. C.) and a Mooney viscosity(ML.sub.1+4, 100.degree. C.) of 40-100; and 5-15 wt.% of talc having an average particle size of 0.5-5 .mu.m. On the other hand, Japanese Patent Laid-open No. 70141/1982 discloses a propylene-base polymer composition consisting of (a) 30-85 wt.% of a crystalline propylene polymer; (b) 5-50 wt.% of an ethylene-propylene copolymer rubber; and (c) 5-30 wt.% of precipitated or wet-ground heavy, fine calcium carbonate particles having an average particle size of 0.05-1.0 .mu.m.
Such resin compositions containing impact resistance improvers incorporated therein are however accompanied by drawbacks. Their stiffness, heat resistance and molding fluidity have been lowered although their impact resistance and paintability have certainly been improved.
It is also commonly practiced to add an inorganic filler with a view toward improving the stiffness of polypropylene resin. According to this method, the stiffness is enhanced and the heat resistance is improved as the proportion of such an inorganic filler is increased. However, it has been known that an addition of such a filler, on the other hand, leads to a considerable reduction in impact resistance. For example, it is disclosed in "Engineering Materials", Vol. 20, No. 7, Page 29 (1972) that the Izod impact strength of polypropylene decreases as more talc is added although its stiffness and heat distortion temperature are improved. It is also disclosed in "Plastics", Vol. 17, No. 12, Page 27 (1966) that the Izod impact strength of polypropylene is lowered when diatomaceous earth, calcium carbonate, talc or asbestos is added thereto. Incidentally, the inorganic fillers used in the abovementioned methods are of those employed routinely and no special attention has been paid to their particle sizes.